Base station oscillator tuned with received clock signal

Abstract

To generate a carrier frequency, a base station needs an oscillator with a long-term stability better than 5·10 -8 . In the case of crystal oscillators it is therefore necessary to regularly adjust the frequency so as to compensate for frequency drift due to aging. The oscillator (OSC) of a base station (BTS) is automatically tuned with the pulse repetition frequency of the clock (CLK) of a digital communication signal (STM). To prevent variations in the pulse repetition frequency of the clock (CLK) from affecting the stability of the carrier frequency (CAR), the oscillator (OSC) is only tuned during times that a quality indicator contained in the digital communication signal (STM) guarantees a high frequency accuracy of 10 -11 .

I claim: 
     
       1. A base station (BTS) with a tunable oscillator (OSC) for generating a carrier frequency (CAR) and with an arrangement (VA) for tuning the oscillator (OSC), characterized in that the arrangement (VA) for tuning the oscillator (OSC) comprises: a clock recovery unit (TA) for deriving a clock signal (CLK) having a pulse repetition frequency from a digital communication signal (STM);   a frequency comparator (FC) for generating a correction signal (COR) dependent on a difference between a frequency (OF) of the oscillator (OSC) and a pulse repetition frequency of the clock signal (CLK), said correction signal (COR) serving to tune the oscillator (OSC); and   a control facility (LOGIC) for tuning the oscillator (OSC) with the correction signal (COR) in accordance with a quality indicator which indicates stability of the clock signal and forms part of the digital communication signal (STM).   
     
     
       2. A base station (BTS) as claimed in claim 1, characterized in that the control facility (LOGIC) is provided for evaluating the quality indicator, which is contained in a frame of the digital communication signal. 
     
     
       3. A base station (BTS) as claimed in claim 1, characterized in that the digital communication signal is a pulse-code-modulated (PCM) signal which is received by a mobile switching center from a communications network. 
     
     
       4. A base station (BTS) as claimed in claim 1, characterized in that the digital communication signal (STM) is of a synchronous digital hierarchy, that as the quality indicator, the control facility (LOGIC) uses bits 5 to 8 of an S1 byte, which is contained in a section overhead (SOH) part of the frame of the digital communication signal (STM), and that the oscillator (OSC) is tunable only when the quality indicator consists of a bit sequence 0010. 
     
     
       5. A base station as claimed in claim 1, characterized in that the digital communication signal (STM) is of a plesiochronous digital hierarchy, that as the quality indicator, the control facility (LOGIC) uses one of bits S a4  to S a8  in a superframe structure of the digital communication signal (STM), and that the oscillator (OSC) is tunable only when the quality indicator consists of a bit sequence 0010. 
     
     
       6. A base station as claimed in claim 1, characterized in that the clock recovery unit (TA) includes a low-pass filter which suppresses phase and frequency variations of the clock signal (CLK) in the digital communication signal (STM) with a time constant of up to several hours. 
     
     
       7. A method of tuning a tunable oscillator (OSC) for generating a carrier frequency (CAR) of a base station (BTS), comprising the steps of: deriving a clock signal (CLK) having a pulse repetition frequency from a digital communication signal (STM);   generating a correction signal (COR) dependent on a difference between a frequency (OF) of the tunable oscillator (OSC) and the pulse repetition frequency of the clock signal (CLK); and   tuning the oscillator (OSC) with the correction signal (COR) in accordance with a quality indicator which indicates stability of the clock signal and forms part of the digital communication signal (STM).   
     
     
       8. A method as claimed in claim 7, characterized in that the digital communication signal (STM) is a communication signal of a synchronous digital hierarchy, that the quality indicator is composed of bits 5 to 8 of an S1 byte, which is contained in a section overhead (SOH) part of a frame of the communication signal (STM), and that the step of tuning the oscillator (OSC) takes place only during times that the quality indicator consists of a bit sequence 0010. 
     
     
       9. A method as claimed in claim 7, characterized in that the digital communication signal (STM) is a communication signal of a plesiochronous digital hierarchy, that the quality indicator uses one of a plurality of bits S a4  to S a8  in a superframe structure of the communication signal (STM), and that the step of tuning of oscillator (OSC) takes place only during times that the quality indicator consists of a bit sequence 0010.